1. Field of the Invention
The invention relates to a method for identifying substances that are capable of acting as anesthetics.
2. Background of the Related Art
Volatile anesthetics are a remarkable class of agents producing a safe, reversible state of unconsciousness with concurrent amnesia and analgesia. They have hyperpolarizing action on mammalian neurons. They activate an inhibitory synaptic K+ current (IK(An)) in molluscan pacemaker neurons which has been proposed to have an important role in general anaesthesia.
Volatile anesthetics hyperpolarize frog motor neurons, rat hippocampal neurons, guinea pig thalmic neurons and human cerebral cortex neurons. Therefore, it has been proposed that the molecular mechanism of volatile anesthetics involves an action on a specific class of K+ channels. The fact that a particular inhibitory synaptic K+ current, IK(An), reversibly activated by volatile agents is present in anesthetic-sensitive molluscan pacemaker neurons, but absent in insensitive neurons has made it a very attractive candidate as a target for these important pharmacological agents. IK(An) behaves as a background channel; it is not voltage-gated, it activates immediately, and it does not inactivate with time. IK(An) obeys the Goldman-Hodgkin-Katz constant field equation and is resistant to the classical K+ channel blockers, tetraethylammonium and 4-aminopyridine.
We recently identified a novel family of mammalian K+ channels with a unique structural motif consisting of two pore domains in tandem and four transmembrane segments. The four members in this family have been classified as TWIK-1, TASK, TREK-1, and TRAAK, which are shown in FIG. 1A. TWIK-1 has been previously shown to dimerize, implying that a functional channel is formed by at least two subunits. Heteromultimerization does not occur between the four members of this novel family as tested in Sf9 cells expressing various combinations of these channels (unpublished data). Three members of this family, mouse TREK-1, mouse TRAAK, and human TASK, encode for background outward-going K+ rectifiers with properties resembling those of IK(An). TRAAK and TREK-1 are directly opened by arachidonic acid and other polyunsaturated fatty acids, while TASK encodes a resting K+ channel which is controlled by external pH variations near physiological pH. TREK-1 and TASK are expressed in many tissues and are particularly abundant in the brain and in the heart, whereas TRAAK is selectively expressed in the central nervous system. Neuronal expression of these channels is detected at high levels in the cortex, cerebellum, hippocampus and olfactory bulb, and cardiac expression is detected in both the myocardium and connective tissues.